Touch screen with relatively conductive grid

ABSTRACT

A new touch screen design provides for corrections of non-uniformities, more accurate touch point measurement, and multiple simultaneous touch point measurements through the use of a grid of relatively conductive lines.

INTRODUCTION

Since their introduction in the early 1970s, touch screens have affordedattractive alternatives to keyboards for certain computer applications.In many situations the keyboard and mouse are eliminated, because thetouch screen provides the user with a much easier access to thecomputer. As a consequence, the market has grown to a substantial size,and a continued rapid growth is anticipated. However, current touchscreens are difficult to produce, which creates a price barrier limitinggrowth into many new areas, such as education.

In this disclosure, a new concept is discussed that virtually eliminatesdesign constraints and provides more freedom for the configuration oftouch screens. Examples are given to illustrate this new freedom indesign parameters. These design concepts provide a basis for producingtouch screens at a much lower cost, without sacrificing quality.Furthermore, the creation of new designs for special sensor size, shape,or electrical characteristics is greatly simplified with the conceptdescribed herein and reduces research and development costs.

BACKGROUND OF THE INVENTION

Touch screens based on materials that conduct electricity uniformly havebeen in use for several decades. One of the first touch screens was madefrom two sheets of conductive paper so arranged that two independentelectrical fields ran orthogonal in a steady state (Hurst and Parks,U.S. Pat. No. 3,662,105). Later improvements consisted of using an arrayof highly conductive dots as electrodes around the rectangular perimeterand a complementary array of diode switches, or preferably a resistornetwork, such that only one conductive sheet was required. Potentialswere then measured in a timed sequence (Hurst, U.S. Pat. No. 3,798,370)to obtain both an x and a y coordinate. This development, along with aconducting and transparent ‘cover sheet’ with radius of curvaturediscrimination (Hurst and Colwell, U.S. Pat. No. 3,911,215) helped pavethe way to transparent touch screens that could be used on the computerterminal (Talmage et al, U.S. Pat. No. 4,071,689 and Gibson et al, U.S.Pat. No. 4,220,815).

When diode switches were replaced with voltage dividers using fixedresistors connected to a pattern of highly conducting dots, theswitching required to sample the two time sequenced electrical fieldswas greatly simplified. This concept evolved into the use of a carefullydesigned frit pattern in order that equipotentials could berepresentative of Cartesian coordinates (Talmage et al, U.S. Pat. No.4,797,514) even near the edges of the screen. Recently a borderedelectrode design was introduced (Hurst, et al. U.S. Ser. No. 09/262,909,now U.S. Pat. No. 6,650,319, issued Nov. 18, 2003) to greatly simplifythe production of touch screens. This design consists of a narrow borderthat encloses the working area and is made of a material that isintermediate in electrical conductivity between the highly conductiveelectrodes and that of the sensor coating.

The present disclosure relates to a touch screen whose rectangular areais enclosed with a border that is divided into a number of smallerrectangular areas using lines of specified width and electricalconductivity. With this technique the coating uniformity requirement isreduced. Essentially, uniformity requirements apply not to the entirearea of the touch screen but to the smaller areas defined with the gridof conducting lines. With this innovation, the construction of largetouch screens, even wall size or floor size, can be accomplished. Thiswould make possible a number of new applications such as interactionwith image projection equipment, input information for robots, positionsensitive information for security, or inputs for virtual realityequipment.

With modern electronics technology, it is economically feasible to applycorrections to data from touchscreens with non-uniform fields to obtainaccurate Cartesian coordinates. For example, Hurst, et al. U.S. Ser. No.09/262,909 has shown that topological mapping can be advantageously usedto build resistive touch screens with relaxed uniformity requirementsmuch more economically, without the loss of performance. In thistopological method equipotential pairs are mapped to a pair of Cartesiancoordinates even under conditions where individual equipotentials do notmap to give unique x and y coordinates. However, with some electrodegeometry (for example a spot electrode at each corner of a rectangle)equipotential pair measurements on some regions of the sensor cannot beuniquely mapped to Cartesian coordinates. The use of the border conceptprovides unique pair mapping over the entire working area of the sensor,even very close to the edges.

The present invention provides a grid arrangement that makes more screenarea available even without electronic data correction. However withextremely large screens which might be prepared with large individualsensing areas defined by grids, gross non-uniformity would be expectedand, when necessary, electronic mapping may be applied.

SUMMARY OF THE INVENTION

It is therefore a purpose of the invention to provide an improved touchscreen allowing improved screen yield with inherent tolerance forindividual and lot variances. It is a further object of the invention topermit simplified manufacture requirements for touch screens includingless-demanding conductive-coating application; fewer electrodes and nodivider resistors. It is yet another purpose of the invention to permitmanufacture at low additional cost, more than offset by savings inscreen manufacture. It is another purpose of the invention to permitliberated design of touch screens with changes readily implemented toaccommodate new larger screen configurations.

In one aspect, the invention provides a touchscreen position sensor thatcomprising:

-   -   (a) a touch area having generally relatively low conductivity;    -   (b) a relatively high conductivity border defining a perimeter        on the touch area;    -   (c) a grid of intermediate conductivity defining cells of        relatively low conductivity within the high conductivity        perimeter of the touch area;    -   (d) a set of electrodes attached to the high conductivity        perimeter and connected to an electrical power source;    -   (e) an electric circuit that measures the potential of a        selected point on the touch area when brought in proximity        thereto;    -   (f) a controller that sequentially switches electrical power        from the power source to a first group of the electrodes thereby        establishing a first electrical potential distribution on the        touch area, and then to a second group of said electrodes        thereby establishing a second electrical potential distribution        on the touch area; and    -   (g) a controller that processes sets of potential measurements        of said first and second electrical potential distributions from        the electric circuit to determine the location of the selected        point.

The sensor of the first aspect may have a controller that selectssubsets of a first set of cells to poll a subset of the cells, and thepolling of subsets of cells may permit the determination of more thanone point of simultaneous contact with the touch area. The sensor of thefirst aspect may have a relatively high conductive border with aconductivity at least ten times greater than the conductivity of therelatively lower conductive touch area and the grid of intermediateconductive material may have a conductivity at least four times greaterthan the relatively low conductivity touch area.

In a second aspect, this invention provides a touchscreen positionsensor comprising:

-   -   (a) a touch area having generally relatively low conductivity;    -   (b) a relatively high conductivity border defining a perimeter        on the touch area;    -   (c) a grid of intermediate conductivity defining cells of        relatively low conductivity within the high conductivity        perimeter of the touch area    -   (d) a set of electrodes attached to the high conductivity        perimeter and connected to an electrical power source;    -   (e) an electric circuit that measures the current received by an        electrode from a selected point on the conductive area when the        point is touched; and    -   (f) a controller that processes sets of current measurements        from the electrodes to determine the location of the selected        point.

In a third aspect, this invention provides a method of determining thelocation of a selected point on a sensor apparatus having a relativelyhighly conductive border surrounding an area of relatively lowconductivity featuring a grid having a conductivity intermediate theconductivity of the border and low conductivity area, defining cells onthe low conductivity area, and an associated set of electrodes throughwhich electrical current may be applied to establish electric potentialdistribution on the conductive area, an electrical circuit that measuresthe potential of points on the conductive area when brought in proximitythereto and a controller, comprising the steps of:

-   -   (a) introducing an electrical current to the border area through        a first group of at least one electrode selected from said        associated set of electrodes, and thereby establishing a first        electric potential distribution within the border;    -   (b) bringing the electrical circuit in proximity with a selected        point within the border and thereby measuring a first potential        reading of said point;    -   (c) storing said first potential reading;    -   (d) introducing an electrical current of the border area through        a second group of at least one electrode selected from said        associated set of electrodes and    -   wherein not all of said first and second groups of electrodes        are identical, and thereby establishing a second electric        potential distribution within the border;    -   (e) measuring the second potential reading of the selected point        with the electrical circuit; and    -   (f) processing said first and second potential readings to        determine the location of the selected point.

The method of the third aspect may provide that the controllersequentially switches electrical power from the power source to a firstsubset of the electrodes thereby establishing a first subset electricalpotential subset distributed within a first subset of cells as the toucharea, and thereto a second subset of said electrodes therebyestablishing a second subset of cells on the touch area, the controllerprocessing the sets of potential measurements of said first and secondsubset electrical potential distributions to more precisely determinethe location of the selected point within the cells of the intersectionof the first and second subsets of cells. Alternatively, the method mayprovide that the first electric potential distribution defines a firstset of potential lines within the border and the second electricpotential distribution defines a second set of potential lines withinthe border, and said first and second sets of potential linessubstantially intersect. Furthermore, the method may provide that thelocations of multiple points of simultaneous contact are determined bypolling separate subsets of cells, each subset of cells being selectedto contain only one point of contact.

In a fourth aspect, this invention provides a touch sensor having atouch area of generally low conductivity defined by a border ofrelatively high conductivity wherein the touch area is divided intocells by lines of intermediate conductivity. In this aspect, the touchsensor the conductivity of the low conductivity touch area may not beuniform or the conductivity of the low conductivity touch area within asingle cell may be substantially uniform. Alternatively, the lines ofintermediate conductivity may be sufficiently close together that thelow conductivity touch area within a single cell is substantiallyuniform.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the equipotentials for a grid having the same conductivity(inverse resistance per square) as the bordered frame;

FIG. 2 shows the equipotentials for a grid having a conductivity onlyone-fiftieth ( 1/50) of the bordered frame and twice the conductivity ofthe remaining surface area;

FIG. 3A shows the effect of a grid on reducing the requirement foruniformly conductive coatings when employed with the method of currentinjection on a substrate having a relatively uniform conductivity;

FIG. 3B shows the addition of a grid having ten times the conductivityof the designated rectangular portion of the screen in a currentinjection touch screen;

FIG. 4A shows the entire screen with conductivity of 1 except for thedesignated rectangle having a conductivity of 0.1 and the equipotentialsare distorted around that rectangle;

FIG. 4B shows the same screen as FIG. 4A except with the addition of agrid of intermediate conductivity which restores the equipotentials tovalues close to those achievable with a perfectly uniform coating;

FIG. 5 illustrates the potential distribution with currents beinggenerated in the individual grid lines that are biased when contact ismade between the cover and bottom grid surface;

FIG. 6 shows a diagram of a touchscreen position sensor having anelectric circuit that measures the potential of a point on a touch area;and

FIG. 7 shows a diagram of a touchscreen position sensor having anelectric circuit that measures current received by an electrode from apoint.

DETAILED DESCRIPTION OF THE INVENTION

Computer simulations of the salient features of the grid invention havebeen made, with programs that solve boundary-value problems.Mathematical methods of solving equations of continuity coupled with ageneral relationship between current and electric field are used. Theseequations work for arbitrary conductivity and reduce to Laplace'sequation for uniform conductivity. This general mathematical treatmentleads to solutions for the equipotentials on conducting planes for anyprescribed boundary conditions. For instance they can provideequipotential solutions for geometry where bordered frames are held atstationary electrical potentials, as in a resistance touch screen. Asanother example, solutions for the currents at the four cornerelectrodes when current is injected at a touch point in a rectangularsensor bounded by the bordered frame can be found. These so-calledcurrent injection methods have been used in physics research and fortouch screen production for some time. A recent patent application byBabb and Wilson (PCT, publication number 97/34273) describes the methodand gives references to some earlier developments. These bordered framegeometries can be modified to incorporate grids to show the advantagesof such innovations.

FIG. 1 shows the steady-state equipotential solutions for a grid inwhich the bordered frame is simply extended to the interior regions tocreate 16 discrete areas or cells. This geometry in which the frame andthe interior lines have the same width and electrical conductivity isnot successful as a touch screen, without topological mapping, sincethere is excessive distortion of the equipotential lines. All of theframing materials and the interior grid lines have an electricalconductivity that is 500 times that of the coating (taken to be 1).

FIG. 2 applies to a grid in which the interior lines have a conductivitythat is reduced by a factor of 50 over those in FIG. 1. Thus threevalues of conductivity, 500 for the frame, 10 for the grid lines, and 1for the coating, are involved. It is seen that the equipotentials aredramatically improved by this combination of conductivities, when usedin the grid concept.

Due to some restrictions in the computer-simulation program, accuratesolutions for very narrow lines cannot be obtained. For this reason, itis convenient to work with rather wide grid lines and then to scale theresults to narrow lines of higher conductivity. There is a very simplerule for this scaling. To keep the same resistance on the grid lines,the scaling law is that the product of conductivity with the width ofthe line should remain constant during the transition. For instance,good equipotentials were found by using the prescribed conductivity. Forconvenience in production it would be much better if all of the linesmaking up the grid have the same conductivity. If the lines have a widthof 0.2 inches, the equipotential quality is maintained for a line widththat is reduced by the factor of 50 to 0.004 inches (4 mils), where allof the lines now have the same conductivity. This is very fortunatesince the visual appearance of the screen is improved by having veryfine lines. However, it is possible to make conducting coatingstransparent to light, and in this case lines of appropriate conductivitycan have a range of widths.

FIGS. 3 a and 3 b apply to the method of current injection, and show theeffect of the grid on reducing the requirement for uniformly conductingcoatings. This was done by taking the conductivity to be 0.1 in thedesignated rectangular portion of the screen. In FIG. 3 a, the solutionfor the equipotentials, from which the four corner currents can bederived and processed to obtain Cartesian coordinates, is ratherasymmetric. The ‘4 mil’ grid, providing a ten-fold increase inconductivity around the same region in FIG. 3 b, gave much betterbehaved equipotentials and when processed gave Cartesian coordinatesthat were much closer to the correct values.

The advantage of using the grid is much easier to illustrate with themethod in which the grid is held at steady-state potentials everywhereon the sensor and coordinates are found just by measuring a pair ofpotentials at some place on the screen. With the same construction as inFIG. 3, we show these results in FIGS. 4 a and 4 b. Thus even for therather drastic reduction of conductivity to 0.1 within the designatedregion, the grid does a good job in restoring the equipotentials to thevalue they would have had with a perfectly uniform coating.

While the original concept of the grid was to enable the manufacture ofvery large screens, it is also useful to try to extend the technology tosolve a basic problem with that has been common to all touch screenssince their inception. This is the problem of ‘multiple touch.’ Imaginethe use of touch screens in a robotics application, where it might bedesirable to give information to a robot on the location of a number ofdifferent objects on a table. Or take another application where it mightbe desirable to know the location of individuals as they walk on afloor. In either case the technical problem is the same—namely, thereare multiple contacts on the touch screen; whereas the usual touchscreen is designed to be touched at only one point at a time.

To approach the solution of this problem, imagine the touch screen to bedivided into cells with grid lines as described above. But beforeproceeding, also imagine another use of the grid and this will serve asa basic element of the solution of the multiple contact problem.Therefore, first it is desirable to describe a touch screen that can beused to zoom into smaller areas, thus to obtain greater resolution andaccuracy of measurements of the location of a single touch point.

The idea is implemented by using a grid that is first configured in thestandard way with voltages applied to the four corners and then switchedin a timed sequence to estimate a coordinate pair (x, y). After a gridcell has been located in this way, the corner voltages are applied totwo lines of x and two lines of y so that the appropriate cell is nowsingled out of the matrix of a large number of cells; the standardmeasurement sequence is repeated to obtain a much more accurate estimateof the point (x, y).

Sequential interrogation in large applications involving fast movingmultiple contacts may, in some cases, be too slow. It is thereforehelpful to develop algorithms to permit simultaneous interrogation ofgroups of grid cells so that no more than 8 or 16 current cycles arenecessary before the interrogation is complete.

Another solution to the cell interaction problem is to divide theconducting cover sheet commonly used to contact the sensor surface intosegments that match the grid cells. This requires the use of insulatingstrips on the cover sheet and the use of connecting lines from eachsegment to a matrix switch. In this way all of the cells with a singletouch point could be counted and the position within each cell could beapproximated. The zoom procedure described above could be applied in atimed sequence to obtain the location of the touch point on each of thesegments with improved resolution. In other words, the location of eachof the multiple touch points can be obtained with good resolution,provided that each cell has only one touch point.

The segmented cover sheet greatly simplifies the interrogation of eachcell but seems to require a row-column matrix to activate individual orgroups of cells. This can be accomplished with some burden on themanufacturing. Successful interrogation or polling of the screen mayalso be possible without segmenting the cover screen. The burden isplaced on the computation.

For example, if the floor of a room has grid with cells about 1-2 feeton a side, then the locations of several people might be discerned byserial polling and data interpretation. Adjacent pairs of grid elementscould be biased to receive current from those standing within (oradjacent to) that row or column. When all pairs of row and column gridlines have been serially biased, the set of data could be analyzed tointerpret the number and locations of the people standing (or walking).FIG. 5 illustrates the potential distribution; if contact is madebetween the cover and bottom grid surface, then currents are generatedin the individual grid lines that are biased.

FIG. 6 shows a touchscreen position sensor in which touch screen 10containing a touch area is electrically connected via electrodes 12 topower source 14. Controller 16 contains circuit 18 that measures thepotential of a point on a touch area by means of switches 20. Also shownare coversheet 22 and storage for potential readings 24.

FIG. 7 shows a touchscreen position sensor in which touch screen 10 iselectrically connected via electrodes 12 to power source 14. Controller16′ contains current measuring circuit 26 that measures current receivedby an electrode from a point via current measurement elements 28.

The present invention liberates the design of sensors for touch screenapplications. Furthermore, this versatility comes with great simplicityand with no sacrifice of quality.

The invention relies on the fact that the surface area of a touchscreencan be divided into any number of smaller areas using lines of specifiedwidth and electrical conductivity. The smaller areas therefore dividethe touchscreen into cells that need only be uniform for that particularcell. The conductive borders of each cell tend to normalize some of thenon-uniformity within the cell. Furthermore, with this innovation, theconstruction of large touch screens can be accomplished. Without sizerestrictions, touchscreens have a number of new applications that werepreviously unavailable such as interaction with image projectionequipment, position sensitive information for security, or inputs forvirtual reality equipment.

In addition to the increasing number of touchscreen technologyapplications, the present invention could actually decrease the cost ofproduction in some cases A major expense of manufacturing touchscreensis the precision required to make uniform touchscreens such that currentmay be distributed evenly along the electrically conductive surface tocreate the necessary equipotential lines.

If any part of the screen becomes non-uniform the screen must bere-processed. The grid decreases the uniformity requirements fortouchscreen surfaces and therefore permits less rigorous manufacturingspecifications. Other cost savings may be realized by lowering the timeand precision required in quality control procedures.

Thus, it is believed that the overall production cost of large touchscreens could be considerably reduced without the loss of quality. Atthe same time, new designs can be implemented without excessiveengineering efforts. The combination of design freedom and the reducedproduction costs should impact the industry in a positive way;especially since there are markets, such as education, personnel use,and home entertainment, that would benefit from large touch screens.

Numerous alterations of the structure and methods herein described willsuggest themselves to those skilled in the art. It will be understoodthat the details and arrangements of the parts that have been describedand illustrated in order to explain the nature of the invention are notto be construed as any limitation of the invention. All such alterationswhich do not depart from the spirit of the invention are intended to beincluded within the scope of the appended claims.

1. A touchscreen position sensor comprising (a) a touch area havinggenerally relatively low conductivity; (b) a relatively highconductivity border defining a perimeter on the touch area; (c) a gridof intermediate conductivity defining cells of relatively lowconductivity within the high conductivity perimeter of the touch area,the relatively high conductivity border having a conductivity at leastten times greater than the conductivity of the relatively lowconductivity touch area and the grid of intermediate conductivity havinga conductivity at least four times greater than the relatively lowconductivity touch area; (d) a set of electrodes attached to the highconductivity perimeter and connected to an electrical power source; (e)an electric circuit that measures the potential of a selected point onthe touch area when brought in proximity thereto; (f) a controller thatsequentially switches electrical power from the power source to a firstgroup of the electrodes thereby establishing a first electricalpotential distribution on the touch area, and then to a second group ofsaid electrodes thereby establishing a second electrical potentialdistribution on the touch area; and (g) a controller that processes setsof potential measurements of said first and second electrical potentialdistributions from the electric circuit to determine the location of theselected point, wherein the controller may select subsets of a first setof cells to poll a subset of the cells.
 2. The sensor of claim 1 whereinthe conductivity of the cells and the touch area are not uniform.
 3. Thesensor of claim 1 wherein the touch area is generally rectangular and anelectrode is located near each corner of said area.
 4. The sensor ofclaim 1 wherein polling of subsets of cells permits the determination ofmore than one point of simultaneous contact with the touch area.
 5. Thesensor of claim 1 wherein the line width of the relatively highconducting border is greater than the line width of the grid ofintermediate conductivity defining cells.
 6. The sensor of claim 1wherein the grid lines are transparent to light.
 7. A method ofdetermining the location of a selected point on a sensor apparatushaving a relatively highly conductive border surrounding an area ofrelatively low conductivity featuring a grid having a conductivityintermediate the conductivity of the border and low conductivity area,defining cells on the low conductivity area, and an associated set ofelectrodes through which electrical current may be applied to establishelectric potential distribution on the conductive area, an electricalcircuit that measures the potential of points on the conductive areawhen brought in proximity thereto and a controller, comprising the stepsof: (a) introducing an electrical current to the border area through afirst group of at least one electrode selected from said associated setof electrodes, and thereby establishing a first electric potentialdistribution within the border; (b) bringing the electrical circuit inproximity with a selected point within the border and thereby measuringa first potential reading of said point; (c) storing said firstpotential reading: (d) introducing an electrical current of the borderarea through a second group of at least one electrode selected from saidassociated set of electrodes and wherein not all of said first andsecond groups of electrodes are identical, and thereby establishing asecond electric potential distribution within the border; (e) measuringthe second potential reading of the selected point with the electricalcircuit; and (f) processing said first and second potential readings todetermine the location of the selected point, wherein the controllersequentially switches electrical power from the power source to a firstsubset of the electrodes thereby establishing a first subset electricalpotential subset distributed within a first subset of cells as the toucharea, and thereto a second subset of said electrodes therebyestablishing a second subset of cells on the touch area, the controllerprocessing the sets of potential measurements of said first and secondsubset electrical potential distributions to more precisely determinethe location of the selected point within the cells of the intersectionof the first and second subsets of cells.
 8. The method of claim 7wherein the first electric potential distribution defines a first set ofpotential lines within the border and the second electric potentialdistribution defines a second set of potential lines within the border,and said first and second sets of potential lines substantiallyintersect.
 9. The method of claim 7 wherein said first and secondreadings are uniquely mapped into spatial coordinates for the selectedpoint.
 10. The method of claim 7 wherein an algorithm is implemented insoftware run on a controller to interpolate the first and secondreadings.
 11. A method of determining the location of a selected pointon a sensor apparatus having a relatively highly conductive bordersurrounding an area of relatively low conductivity featuring a gridhaving a conductivity intermediate the conductivity of the border andlow conductivity area, defining cells on the low conductivity area, andan associated set of electrodes through which electrical current may beapplied to establish electric potential distribution on the conductivearea, an electrical circuit that measures the potential of points on theconductive area when brought in proximity thereto and a controller,comprising the steps of: (a) introducing an electrical current to theborder area through a first group of at least one electrode selectedfrom said associated set of electrodes, and thereby establishing a firstelectric potential distribution within the border; (b) bringing theelectrical circuit in proximity with a selected point within the borderand thereby measuring a first potential reading of said point; (c)storing said first potential reading; (d) introducing an electricalcurrent of the border area through a second group of at least oneelectrode selected from said associated set of electrodes and whereinnot all of said first and second groups of electrodes are identical, andthereby establishing a second electric potential distribution within theborder; (e) measuring the second potential reading of the selected pointwith the electrical circuit; and (f) processing said first and secondpotential readings to determine the location of the selected point. thelocations of multiple points of simultaneous contact being determined bypolling separate subsets of cells, each subset of cells being selectedto contain only one point of contact.